fix: audit fix — reduce password copies in memory (v0.8.4)

- attempt_login takes Zeroizing<String> by value, eliminating the redundant
  Zeroizing::new(password.to_string()) that doubled the Rust-owned copy.
- Clear password_entry's internal buffer immediately after extracting the
  password, shortening the window during which the GTK GString persists in
  non-zeroizable libc memory.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moongreet"
-version = "0.8.3"
+version = "0.8.4"
 edition = "2024"
 description = "A greetd greeter for Wayland with GTK4 and Layer Shell"
 license = "MIT"